Conducting polymer nanofiber sensors

ABSTRACT

Polymer nanofibers, such as polyaniline nanofibers, with uniform diameters less than 500 nm can be made in bulk quantities through a facile aqueous and organic interfacial polymerization method at ambient conditions. The nanofibers have lengths varying from 500 nm to 10 μm and form interconnected networks in a thin film. Thin film nanofiber sensors can be made of the polyaniline nanofibers having superior performance in both sensitivity and time response to a variety of gas vapors including, acids, bases, redox active vapors, alcohols and volatile organic chemicals.

STATEMENT OF GOVERNMENT INTEREST

The invention was made with Government support under contract No.F04701-00-C-0009 by the Department of the Air Force. The Government hascertain rights in the invention.

REFERENCE TO RELATED APPLICATIONS

The present application is related to applicant's copending applicationentitled “Synthetic Method for Conducting Polymer Nanofibers”,10/735,079, filed Dec. 11, 2003, now U.S. Pat. No. 7,144,949 by the sameinventors.

FIELD OF THE INVENTION

The invention relates to the field of nanostructured polymers. Moreparticularly, the present invention is related to methods of manufactureof conducting polymer nanofibers and sensors made of conducting polymernanofibers.

BACKGROUND OF THE INVENTION

Since the discovery that conjugated polymers can be made to conductelectricity through doping, research has been extended in the field ofconducting polymer films. Polymers have been made as conducting links oforganic monomers having defined chemical structures. Polyaniline can bemade as a conducting polymer of aniline monomers. Polyaniline is aunique conjugated polymer in that polyaniline can be tailored forspecific applications through a non-redox acid and base doping process.Polyaniline has been studied for electronic and optical applications,such as lightweight battery electrodes, electromagnetic shieldingdevices, anticorrosion coatings, and sensors. Unlike other conjugatedpolymers, polyaniline has a simple and reversible acid doping and basededoping chemistry enabling control over properties of the polyaniline,such as density, solubility, conductivity, and optical absorption.One-dimensional polyaniline nanostructures, including nanowires,nanorods, and nanotubes possess low-dimensional sizes and organicconduction. The electrically conductive form of polyaniline is known asemeraldine having an oxidation state which, when doped with an acid,protonates the imine nitrogens on the polymer backbone and inducescharge carriers. The conductivity of polyaniline increases with dopingfrom the undoped insulating emeraldine base form, σ<10⁻¹⁰ S/cm, to thefully doped, conducting emeraldine salt form, σ>1 S/cm. Dopants can beadded in any desired quantity until all imine nitrogens, that is half ofthe total nitrogens, are doped, by controlling the pH of the dopant acidsolution. Dopants can be removed by interacting the emeraldine salt formwith common bases such as ammonium hydroxide.

Conducting polymers can be used in sensors having optical,electrochemical and conducting properties. Conducting polymers areunique by changing properties when chemically treated with oxidizing orreducing agents. After chemical treatment with protonating,deprotonating, oxidizing or reducing agents, the conducting polymerpolyaniline can reversibly change from an initially electricallyinsulating state to a conducting state. This transition can be used insuch applications as optical sensors, chemical sensors, and biosensors.Conducting polymers include polyaniline, polypyrole, polythiophene, andtheir derivatives. Polyaniline is a conducting polymer that isenvironmentally stable and can react with chemical species at roomtemperature. As such, polyaniline may be suitable for gas sensingapplications using processes that create a uniform thin film of thepolyaniline. This thin film may then react with protonating anddeprotonating agents to create a conduction pathway that can easily bemeasured.

The conductivity depends on both the ability to transport chargecarriers along the polymer backbone and the ability of the carriers tohop between polymer chains through interpolymer conduction. Anyinteractions with polyaniline that will alter either of these conductionprocesses will affect the overall conductivity. This is the underlyingchemical property enabling polyaniline to be used as the selective layerin a chemical vapor sensor, such as, a resistance detector generallyknown as a chemiresistor. Due to room temperature sensitivity, the baseof deposition onto a wide variety of sensor substrates and due to thevarious structures, conducting polymers are potential materials forsensor applications. A polymer chemiresistor would typically consist ofa substrate, electrodes, and a conducting polymer selective thin film.Changes in conductivity of the polymer film upon exposure to chemicalvapors can be readily monitored with an ohmmeter or electrometer.Polyaniline sensor research has focused on changing the polymerstructure to facilitate interaction between vapor molecules and thepolymer either by modifying the polymer backbone or the interchainconnections. However, poor diffusion can readily outweigh anyimprovements made to the polymer chains because most of the materialother than the limited number of surface sites, is not available forinteracting with a chemical vapor, thus degrading sensitivity. One wayto enhance diffusion is to reduce film thickness, such as producingmonolayers of conventional polymer materials, which leads to a trade-offbetween sensitivity and robustness. Coating polyaniline on poroussubstrates can increase the surface area, but the chemistry and physicsinvolving polymer support and polymer electrode interfaces is not welldefined for practical use.

Nanostructured polyaniline, such as nanowires, nanofibers, nanotubes,and nanorods may have sufficiently high surface area and fasterdiffusion rates of gas molecules into the nanostructures for use aschemical sensors with increased sensitivity, as compared to bulkpolyaniline. For example, the surface area per unit mass S_(A) ofpolyaniline nanofibers increases geometrically as the diameters d of thenanofibers decrease, that is S_(A)˜1/d. Even when the thickness of anultra-thin film is the same as the diameters of the nanofibers, thefibers may outperform a thin film because the fibers have highersurface-to-volume ratios due to their cylindrical morphology. The smalldiameter of the nanofibers, for example less than 500 nm, coupled withthe possibility of gas approaching from all sides should give sensorswith improved performance. Despite the high surface area and porosityassociated with nanostructures, nanostructured polyaniline has not beenused as chemical sensors. This is due to uncertain nanostructurecharacterization as well as the lack of reliable methods to make highquality polyaniline nanofibers, and reliable methods to coat surfaceswith polyaniline nanofibers. No practical nanostructured conductingpolymer sensors are available due to the lack of reliable methods formaking high quality conducting polymer nanostructures in bulk quantitiesand the unknown properties of nanofiber characterization.

Syntheses of polyaniline nanostructures have been carried out bothchemically and electrochemically by polymerizing the aniline monomerswith the aid of either a hard template or a soft template. Examples ofhard templates include zeolite channels, track-etched polycarbonate,nanoporous membranes, and anodized alumina. Examples of soft templatesfor self-assembly of functional polymers include surfactants,polyelectrolytes, or complex organic dopants, such as micelles, liquidcrystals, thiolated cyclodextrins, and polyacids, that may be capable ofdirecting the growth of polyaniline one-dimensional nanostructures withdiameters smaller than 500 nm. Adding structural directing moleculessuch as surfactants or polyelectrolytes to the chemical polymerizationbath is one way to obtain polyaniline nanostructures. When organicdopants with surfactant functionalities are used, emulsions or micellescan be formed leading to microtube, microfiber, or microrod structures.However, when polyaniline nanostructures with diameters of less than 500nm are desired, then very complex dopants with bulky side groups areneeded, such as sulfonated naphthalene derivatives, fullerenes, ordendrimers.

The formation of polyaniline nanostructures disadvantageously relieseither on guidance from hard templates or self-assembled soft templates.These templates disadvantageously use complex synthetic conditions thatrequire the removal of such templates and hence provide low yields andwith poor reproducibility. Chemical methods of making polyanilinenanostructures, such as nanotubes, nanofibers, nanowires, and nanorods,disadvantageously require specific structure-directing templatematerials added into or applied to the polymerization bath. Thesynthetic conditions disadvantageously have to be carefully designed toaccommodate formation and purification to obtain pure polyanilinenanostructures. These template methods are disadvantageously dependenton either a template or a specific complex chemical reagent, andpost-synthetic treatments are needed to remove the reagent from thebyproducts in order to recover pure nanostructured polyaniline.Therefore, developing synthetic production methods that do not rely ontemplates, structural directing molecules, or specific dopants isdesirable, especially for scaling up to produce large quantities ofnanostructured materials suitable for mass usage in chemical sensors.

Electrochemical polymerization and physical methods, such aselectrospinning and mechanical stretching can produce conducting polymernanofibers without templates, but these conducting polymer nanofibermaterials can only be made on carefully prepared surfaces offeringlimited production scaling. Electrochemical synthesis of polyaniline hasindicated that some nanofibers form naturally on a synthesis surfacewhile the underlayer is much more compact with microfiber polymers. Forthe production of polyaniline nanofiber sensors in quantity, thereexists a need for a practical bulk synthetic method. Despite the varietyof current synthetic methods available to produce polyanilinenanostructures, there is a need for a practical synthetic method capableof making pure, uniform, and template-free polyaniline nanostructureswith predetermined small diameters and in bulk quantities. Currentsynthetic methods are not useful in mass production of ultra-small,low-dimensional structures, such as sensors, using conductive polymernanofibers of polyaniline. These and other disadvantages are solved orreduced using the present invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for forming aconductive polymer.

Another object of the invention is to provide a method for formingconductive polymer nanofibers.

Another object of the invention is to provide a method for formingconductive polymer nanofibers with predetermined diameters.

Yet another object of the invention is to provide a method for formingconductive polymer polyaniline nanofibers.

A further object of the invention is to provide a method for forming andpurifying conductive polymer polyaniline nanofibers in a polymerizationbath suitable for production scaling in bulk quantities.

Still another object of the invention is to provide chemical sensorsmade from conductive polymer polyaniline nanofibers.

Yet a further object of the invention is to provide chemical sensorsmade from conductive polymer polyaniline nanofibers having strongadherence to conducting terminals.

The invention is direct d towards methods for producing conductingpolymer nanofibers and chemical sensors made of conducting polymernanofibers. In a preferred form, conducting polyaniline nanofibers areproduced in a polymerization bath, suitable for batch bulk productionand suitable for making a variety of chemical sensors. Polyanilinenanofibers can be doped with an acid and dedoped using a base, in areversible chemical process. Discovery is made that nanostructuredpolyaniline has greater sensitivity and faster chemical time responsesthan the bulk form due to higher effective surface areas and shorterpenetration diffusion depths for gas molecules. Discovery is furthermade that a thiol film on the gold terminals adheres to polyanilinenanofibers for securing a nanofiber polyaniline thin film to a goldconductor. Discovery is further made that polyaniline nanofibers, havingdiameters less than 500 nm and lengths less than 10 μm, in a thin filmhave sufficient conductivity changes in response to dopants anddedopants to be suitable for use in chemical sensors. Discovery is alsomade that selective acids used during polymerization of the polyanilinenanofibers predetermine the resulting diameter of the nanofibers. In thepreferred form, an acid and base chemical vapor sensor can be made usingconventional gold sensor terminals covered by a polyaniline nanofiberthin film.

In the preferred form, polyaniline nanofibers can be uniformly producedwith predetermined diameter sized nanofibers having predeterminedlengths in a normal distribution. During polymerization, a selected acidis used to efficiently facilitate the polymerization process, which isselected to predetermine a normal distribution of diameters of theproduct polymer nanofibers. Each different diameter nanofiber has aresulting different chemical response, and hence, the polymerizationprocess can be tailored to specific performances of the chemicalsensors. These polyaniline nanofibers can then be used in a variety ofchemical sensors, such as acids, bases, alcohols, volatile organicchemicals and reducing agents. The resulting thin film sensors made ofthe nanofibers have superior performance in both sensitivity and timeresponse to a variety of gas vapors. Exemplar acids include hydrochloricacid, sulfuric acid, nitric acid, perchloric acid, and camphorsulfonicacid. Exemplar bases include ammonia and butylamine. Exemplar alcoholsinclude methanol, ethanol, and propanol. Exemplar volatile organicchemicals include chloroform and nitromethane. Exemplar reducing agentsinclude hydrazine.

In the general form, polymer nanofibers can change physical propertiessuch as conductivity, density, conformation, oxidation state, andoptical absorption, among others, offering a wide variety of sensors forsensing various materials. Polyaniline nanofibers with uniform diametershave predetermined response characteristics. The polyaniline nanofiberscan be made in bulk quantities through a facile aqueous and organicinterfacial polymerization method at ambient conditions. The nanofibershave varying lengths within a normal distribution and forminterconnected networks as a thin film of polyaniline. In the generalform, the invention is directed to a synthesis method for producingpolymer nanofibers well suited for use in chemical sensors. In thepreferred form, the method is suitable for bulk production ofpolyaniline nanofibers for use in acid and base chemical sensors. Thesynthesis method is preferably applied to organic monomers that can belinked into conducting polymers, such as polyaniline, polypyrrole,polythiophene and their derivatives. A template-free process using anaqueous polymerization bath provides a practical bulk synthetic methodcapable of producing bulk quantities of pure and uniform nanofibers withsmall predetermined diameters. The synthesis method is based on chemicaloxidative polymerization of the monomers. The polymerization process isperfected in an immiscible, organic-aqueous, two-phase system. Thesynthetic method yields polymer nanofibers with nearly uniformreproducible diameters. These and other advantages will become moreapparent from the following detailed description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a chemical diagram of doped polyaniline emeraldine salt.

FIG. 1B is a chemical diagram of undoped polyaniline emeraldine base.

FIG. 2 is a process flow of a method for synthesizing polyanilinenanofibers.

FIG. 3 is a process flow of a method for forming an acid sensor.

FIG. 4A is a graph of polyaniline nanofiber time response to an aciddopant.

FIG. 4B is a graph of polyaniline nanofiber time response to a basededopant.

FIG. 5 is a process flow of a method for forming a polyaniline nanofiberprecoated sensor.

FIG. 6 is a block diagram of a polyaniline nanofiber precoated sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto FIGS. 1A and 1B, monomers, such as aniline monomers can be linkedtogether to form polyaniline. The polyaniline can be doped with an acid,such as hydrochloric acid HCl dopant, as shown in FIG. 1A, and can bededoped with a base, such as, ammonia NH₃.

Referring to FIGS. 1A, 1B, and 2, and more particularly to FIG. 2, aseven-step method of synthesizing polyaniline nanofibers relies on afacile chemical process to produce high quality polyaniline nanofibersunder ambient conditions using an aqueous-organic interfacialpolymerization.

In step 1, a catalysis solution is first formed from water, an acid, andan oxidizer. The acid is preferably hydrochloric acid HCl. but otheracids may be used, such as sulfuric acid H₂SO₄, nitric acid HNO₃,perchloric acid HClO₄, phosphoric acid H₃PO₄, acetic acid CH₃COOH,formic acid HCOOH, tartaric acid C₄H₆O₆, methanesulfonic acid CH₄SO₃,ethylsulfonic acid C₂H₇SO₃, 4-toluenesulfonic acid C₇H₈SO₃, andcamphorsulfonic acid (CSA). The oxidizer is preferably ammoniumperoxydisulfate (NH₄)₂S₂O₈, but other oxidizers may be used, such asiron chloride FeCl₃ and other peroxydisulfate derivates such as Na₂S₂O₈and K₂S₂O₈. In step 2, a monomer solution is formed from a solution of anonconducting monomer and an organic solvent. In the preferred form, themonomer is aniline, but other carbon-based organic monomers can be used,such as pyrrole, thiophene, toluidine anisidine and other derivatives ofaniline such as methylaniline, ethylaniline, 2-alkoxyaniline, and2,5-dialkoxyaniline monomers, for forming polyaniline, polypyrrole,polythiophene, polytoluidine, polyanisidine, polymethylaniline,polyethylaniline, poly(2-alkoxyaniline) and poly(2,5-dialkOxyaniline)respectively. The organic solvent is preferably carbon tetrachioride(CCl₄), but other organic solvents may be used, such as benzene,toluene, chloroform, methylene chloride, xylene, hexane, diethylether,dichloromethane and carbon disufide. In the preferred form, anilinemonomers are dissolved in carbon tetrachioride (CCl₄). In the preferredform, the monomer is aniline, but other carbon-based organic monomerscan be used, such as pyrrole, thiophene, toluidine, anisidine and otherderivatives of aniline such as methylaniline, ethylaniline,2-alkoxyaniline, and 2,5 dialkoxyaniline monomers, for formingpolyaniline, polypyrrole, polythiophene, polytoluidine, polyanisidine,polymethylaniline, polyethylaniline, poly2-alkoxyanilines andpoly2,5-dialkoxyanilines respectively. The organic solvent is preferablycarbon tetrachloride (CCl₄), but other organic solvents may be used,such as benzene, toluene, chloroform, methylene chloride, xylene,hexane, diethylether, dichloromethane and carbon disufide. In thepreferred form, aniline monomers are dissolved in carbon tetrachloride(CCl₄).

In step 3, the monomer solution is disposed in a reaction vessel thatcan be scaled from small to large for increased batch production of bulkpolymer nanofibers. In step 4, the catalysis solution is disposednonturbulently into the reaction vessel and onto the monomer solutionforming a bifurcated mixture having an aqueous organic reactioninterface between the lower monomer solution and the upper floatingcatalysis solution. In step 5, a polymerization reaction occurs at theaqueous organic reaction interface creating conductive doped nanofiberpolyaniline forming in the upper aqueous catalysis solution. Thecatalysis solution becomes a polymer solution comprising polymers oflinked monomers from the monomer solution. As the catalysis solutionbecomes a polymer solution, the monomer solution is depleted of monomersand becomes an organic solution. Aniline polymerizes at the interfacebetween the bottom organic monomer solution containing dissolved anilineand the upper aqueous catalysis solution containing the oxidant anddopant. As the polymerization reaction proceeds, polyaniline nanofibersform across the interface, slowly diffusing and dispersing into theupper aqueous catalysis solution and eventually filling the upperaqueous solution with dissolved polyaniline nanofibers. At the sametime, the color of the lower organic layer turns red-orange due to theformation of by-products, such as aniline oligomers. The nanofiberproduct in the upper organic solution is then collected and purified byconventional processes such as either dialysis or filtration, yielding ananofiber product in the form of a water dispersion or a powder,respectively. Further washing or dialyzing with water yields pure, dopedpolyaniline, which can be dedoped by washing or dialyzing with a base,such as aqueous ammonia. High quality polyaniline nanofibers withdopants ranging from mineral acids to organic acids can be made. Thesynthesis is readily scalable and can be carried out at roomtemperature.

In step 6, the polymer solution of polymers is separated from theorganic solution. This may be accomplished by siphoning off the topfloating polymer solution, and then disposing of the spent organicsolution. In step 7, the polymer solution is purified into polyanilinenanofibers. The purification step 7 can be accomplished by conventionalfiltration or dialysis methods to effectively extract the polymernanofibers from the polymer solution. For example, the purification canbe by dialysis of the colloidal suspension using standard commerciallyavailable dialysis membranes.

The polymer nanofibers are preferably conducting polymer nanofibershaving predetermined sizes, reactions, and sensitivities topredetermined chemical vapors. The nanofibers have nearly uniformpredetermined diameters dependent upon the specific acid used in thecatalysis solution. The acidic anion has a predetermined size, whichwhen bound to the nanofiber polymer backbone, affect the overalldiameter size of the nanofiber, having typical sizes mostly between 20and 150 nm and less than 500 nm. For example, hydrochloric acid producespolyaniline nanofibers with a 30 nm diameter distribution, CSA acidproduces polyaniline nanofibers with a 50 nm diameter distribution, andperchloric acid produces polyaniline nanofibers with a 120 nm diameterdistribution, all with lengths varying from 500 nm to 5 μm. In thegeneral form, the nanofibers have diameters that are less than 500 nmand lengths less than 10 μm.

Gram scale beaker production to kilogram scale vat production can beused to synthesize various quantities of production polymer nanofibers.The nanofibers are typically twisted together forming an interlockingnetwork or mesh of nanofibers. That is, the nanofibers tend toagglomerate into interconnected nanofiber networks, rather than bundles.Doping and dedoping does not affect the fibrillar morphology. As such,the networks of nanofibers have improved sensitivities, are durable forrepeated use, and are well suited for thin film deposition on sensorterminals. The nanofibers can then be thin film deposited ontoconducting terminals on an insulating substrate to form a sensor.

Referring to FIGS. 1A, 1B, 2, and 3, and more particularly to FIG. 3, anacid chemical sensor can be made using polyaniline nanofibers when amethod of forming an acid sensor having steps 8–13. In step 8, purifiedpolyaniline nanofibers are disposed in a basic water solution forforming an undoped polyaniline nanofiber solution. Discovery is madethat polyaniline nanofibers are stable in water, and can be rapidlydedoped in water using a basic solution. In step 9, the dedopednanofiber polyaniline fibers are purified into a mass of dedopednanofiber polyaniline. In step 10, the dedoped nanofiber polyaniline isdisposed into water for forming a working solution.

In step 11, the conducting terminals are formed on a sensor substrate.The sensor substrate can be made of an insulating material such as glassor quartz. The sensor terminals can be made of a conducting materialsuch as semiconductors and conductors including gold, silver, platinum,polysilicon and doped photoresist. In step 12, the working solution isdisposed onto sensor conducting terminals for coating the terminals withthe nanofibers. This coating step can be accomplished by conventionalcoating methods such as spin coating, drop coating, spray coating, andphotolithography masked deposition coating. In step 13, the workingsolution is dried for coating a film of dedoped polyaniline nanofibersonto the sensor conducting terminals. The nanofibers will then react tothe presence of various chemicals and solutions, such as an acid causinga change in conductivity that can be sensed at the sensor terminals.

Referring to FIGS. 1A, 1B, 2, 3, 4A, and 4B, and more particularly toFIGS. 4A and 4B, polyaniline nanofibers exhibit conductivity changeswhen dedoped or doped polyaniline nanofibers are exposed to an acid orwhen doped polyaniline nanofibers are exposed to a base. In the exemplarcase, when camphorsulfonic acid is used in the catalysis solution, thepolyaniline nanofibers have camphorsulfonic acid molecules are tightlyincorporated within the polyaniline during the in-situ polymerization ofaniline. Dedoped polyaniline nanofibers can be obtained by dialyzing thecamphorsulfonic acid doped polyaniline against 0.1 M ammonium hydroxide,which produces the emeraldine base form of polyaniline. Once thecamphorsulfonic acid molecules are removed, thin-film sensors made ofdedoped fibers respond to repeated doping and dedoping. The sensorperformance of the nanofibers having a predetermined size diameterdistribution, can be measured using a dedoped nanofiber emeraldine basethin film, such as a 2.5 μm thick thin film as compared to aconventional dedoped thin film, such as a 1 μm thick thin film that canbe deposited on an array of interdigitated gold electrodes. The sensoris exposed to an acidic dopant, such as hydrochloric acid, for acidicdoping, and, then exposed to a base, such as ammonia, for basicdedoping. The sensor, for example, may consist of fifty pairs ofelectrodes at 4970×20×0.18 μm on a glass substrate with interdigitatedgaps of 10 μm. The polyaniline nanofiber sensors then exhibit a fast,predetermined reaction time during both doping for acid vapor detectionand dedoping for basic vapor detection. The conducting polyanilinenanofibers possess fast predetermined doping and dedoping times that aresuitable for sensor applications.

The real time resistance changes of a dedoped film can be monitoredusing an electrometer upon exposure to an acid, such as 100 ppm ofhydrochloric acid HCl, as shown in FIG. 4A, and of a fully HCl dopedfilm exposed to a base, such as 100 ppm of ammonia NH₃, as shown in FIG.4B. The resistance changes of a nanofiber emeraldine base thin film andconventional thin film upon exposure to 100 ppm HCl vapor and HCl dopedfilms exposed to 100 ppm NH₃ vapor are shown. The R/R₀ ratio is theresistance R normalized to the initial resistance R₀ prior to gasexposure. The nanofiber thin film responds faster than a conventionalfilm to both acid doping and base dedoping even though the nanofiberfilm is more than twice as thick. This is due to the small,predetermined diameter size of the nanofibers that provides a highsurface area within the film that can be rapidly accessed by the gasvapors. Also, the small, predetermined diameters of the fibers allow gasmolecules to diffuse in and out of the fibers in a much shorterpredetermined amount of time. This also leads to a much greater extentof doping or dedoping over short times for the nanofiber films.

Films of doped nanofiber and conventional polyaniline can be measuredand compared in response to exposure to alcohol vapors, such asmethanol, ethanol, and propanol, or to water vapor. The mechanism ofresponse in the case alcohol or water exposure is not doping or dedopingbut rather conformational changes of the polymer film. Theconformational changes produce a resulting change in conductivity foruse as an alcohol or water vapor sensor. Again, the nanofiber sensor hasa greater response with a faster response time. In the case of volatileorganic chemicals, the response mechanism is swelling of the polymerboth in bulk film and in nanofiber forms. The swelling also causes aresulting change in electrical conductivity, which can be used to sensethe organic vapor. Reducing agents such as hydrazine react withpolyaniline nanofibers to cause a change in oxidation state and acorresponding change in electrical conductivity and can be used to sensehydrazine. The nanofibers can also react with chemical vapors with achange in optical absorption qualities that can be detected using coatedoptical detectors in the chemical sensors. In all cases, the nanofiberfilms have improved sensor performance in both sensitivity magnitude andtime response as compared to conventional bulk films for a range ofanalytes.

Referring to FIG. 5, an enhanced sensor can be made by precoating thegold terminal of a sensor with polyaniline prior to a thin film in amethod described in steps 14 through 24. In step 14, conducting goldterminals are disposed on a sensor substrate. In step 15, the goldterminals are exposed to 4-Amino Thiophenol (4-ATP) for forming a thiolRSH surface layer on the gold terminals. In step 16, the catalysissolution is formed of water, acid, and oxidizer. In step 17, the monomersolution is formed of the monomer and an organic solvent. In step 18,the monomer solution is disposed in a reaction vessel. In step 19, thesensor substrate is submerged in the vessel with the gold terminalspositioned at the surface of the monomer solution. In step 20, catalysissolution is disposed onto the monomer solution forming an aqueousorganic reaction interface flush with the gold terminals. A precoatingof polyaniline nanofibers is formed over upon the thiol RSH surfacelayer while polyaniline nanofibers are also formed at the interface anddispersed into the catalysis solution. In step 21, the polymerizationreaction is continued at the aqueous organic reaction interface creatingconductive, doped polyaniline nanofibers that precoat the gold terminalswhile creating conductive, doped nanofiber polyaniline in the catalysissolution then becoming a polymer solution as the monomer solutionbecomes an organic solution. Alternatively, the polymerization reactioncan also be continued to create a polyaniline nanofiber film thatresults in a conducting nanofiber film that spans electrodes to providea complete chemical sensor after washing of the resulting film. In step22, the polymer solution is separated from the organic solution, whilethe precoated substrate is removed from the vessel. In step 23, thepolymer solution is purified into polyaniline nanofibers. In step 24, afilm of the polyaniline nanofibers is disposed over the precoated goldterminals for forming a precoated sensor as shown in FIG. 6.

Referring to FIG. 6, a polyaniline nanofiber precoated acid sensorincludes the insulating substrate, at least two gold terminals,including a positive gold terminal and a negative gold terminal, uponboth of which is disposed a thiol (RSH) film and a polyaniline precoat.Over the precoated gold terminals is disposed the thin film ofpolyaniline nanofibers. The precoating offers enhanced adherence andcontact conductivity between the gold terminals and the polyanilinefilm. When the polyaniline film is exposed to a chemical vapor, theconductivity of the polyaniline film changes producing changes inresistivity that would then produce a change in electrical current asmeasured by a current meter that is connected in series with a DC powersource that is in turn connected across the positive and negative goldterminals.

Referring to all of the figures, the synthesis method is based on thechemical oxidative polymerization of a monomer, such as aniline, in astrongly acidic catalysis solution having an acid component, such ascamphorsulfonic acid, and an oxidant, such as ammonium peroxydisulfate.The polymerization is performed in an immiscible organic-aqueousbiphasic system, in order to separate the by-products, includinginorganic salts, and oligomers, according to solubility in the organicand aqueous phases. In an exemplar synthetic process, aniline isdissolved in an organic solvent, such as CCl₄, benzene, toluene, or CS₂while ammonium peroxydisulfate is dissolved in water withcamphorsulfonic acid. The two solutions are transferred into a reactionpolymerization vessel for generating an interface between the twosolutions. After a short period, such as a few minutes, greenpolyaniline forms at the interface and then gradually diffuses anddisperses into the aqueous phase of the catalysis solution. After anextended period, such as several hours, the entire water phase ishomogeneously filled with dark green polyaniline, while the lowerorganic layer appears red-orange, due to the formation of anilineoligomers. The aqueous phase is then collected and the by-products areremoved by dialysis against deionized water, using for example, tubingwith a 12K to 14K cutoff. When a deionized water bath reaches a pH of 7,a 10-μl suspension is diluted with 1 ml of deionized water. Dedopedpolyaniline can then be obtained by dialysis using 0.1 M ammoniumhydroxide and then deionized water. The synthetic method produces yieldsranging from six to ten weight percent of nanofibers. Thin film sensorsmade of the polyaniline nanofibers have superior performance in bothsensitivity and time response to a variety of gas vapors including acidssuch as hydrochloric, hydrofluoric, sulfuric, and nitric, bases such asammonia and butylamine, and alcohols such as methanol, ethanol, andpropanol. Thin film sensors made of the polyaniline nanofibers are alsosensitive to some volatile organics such as chloroform and nitromethane,and to redox agents such as hydrazine.

The aqueous and organic interfacial synthesis of polyaniline nanofibershas several advantages. Both the synthesis and purification steps aretemplate free. Uniform nanofibers are obtained in high yields. Thesynthesis method is scalable and reproducible with predeterminedreactions and response times. Multiple syntheses performed frommillimolar to molar quantities produce nanofibers of the samemorphology, size distribution and uniformity. The nanofibers are readilydispersed in water, which facilitates environmentally friendlyprocessing and biological applications. The nanofiber sensors haveshorter doping and dedoping times with greater response to acidic andbasic gases. The nanofiber sensors have a faster and larger response toalcohol vapors and react to some volatile organics and redox agents.Chemical sensors made from the nanofibers respond with larger magnitudesand faster response times to a wide range of analytes, and are wellsuited for chemical sensor applications.

The present invention is generally directed to a synthesis method forforming polymer nanofibers. When the polymer nanofibers are conductive,the resulting thin film can be used in a chemical sensor. Conductingpolymer nanofibers include polyaniline nanofibers, polypyrrolenanofibers, and polythiophene nanofibers all of which can be used inchemical sensors. The method relies on the use of a monomer solutioncomprising an organic monomer and an organic solvent, and the use of acatalysis solution comprising an acid and an oxidizer, for producing atan aqueous and organic interfacial interface polymer nanofibers thatpreferably react to vapor exposures for suitable use in chemicalsensors. Various monomers, solvents, oxidizers, and acids can be used asalternatives, modifications, and improvements to the preferred forms.Those skilled in the art can make enhancements, improvements, andmodifications to the invention, and these enhancements, improvements,and modifications may nonetheless fall within the spirit and scope ofthe following claims.

1. A sensor for sensing the presence of a chemical vapor, the sensoradapted for interconnecting to an electrical monitor for measuring areaction of the sensor to the chemical vapor, the sensor comprising, apositive terminal, the positive terminal being conductive, a negativeterminal, the negative terminal being conductive, the terminals adaptedfor interconnection to the electrical monitor, and a film of organicconductive polymer nanofibers extending between the positive andnegative terminal for producing a change in conductivity between thepositive terminal and the negative terminal as monitored by theelectrical monitor when the film is exposed to the chemical vapor,wherein the positive terminal and the negative terminal are made of goldand the conducting polymer is polyaniline, the sensor furthercomprising, a thiol surface layer disposed between the terminals and thefilm.